image/core-image: Handle conflicting IMAGE_FEATURES.

image/core-image: Handle conflicting IMAGE_FEATURES.

IMAGE_FEATURES such as 'ssh-server-dropbear' and 'ssh-server-openssh'
can't be both enabled. User can use the following variables to define
the relationship of image features:

IMAGE_FEATURES_REPLACES_foo = "bar" means including image feature "foo"
would replace the image feature "bar".

IMAGE_FEATURES_CONFLICTS_foo = "bar" means including both image features
"foo" and "bar" would cause an parsing error.

(From OE-Core rev: e36d12a9c1cf69540079e48a1dfadbc343758e48)

Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>